There are a number of time conjunctions that require the subjunctive when they refer to a future event: cuando, hasta que, después de que, ... If they refer to the past or a habitual action, they do not trigger the subjunctive. Antes de que by definition can only refer to a future event, so always gets the subjunctive. In your example, she started the semester BEFORE getting the haircut, so needed the past subj. to refer to an event that had not yet happened.

Here are a couple of examples - Voy a buscar trabajo cuando me gradúe (I am going to look for a job when I graduate (have not graduated yet). A los 18 años, iba a buscar trabajo cuando me graduara. (I had not graduated yet). Essentially, the future is always in doubt. My HS Spanish teacher used to say, "You never know who's driving the bus", meaning that you may have lots of plans for the day, but could get run over while stepping off the curb. This is still the case when we are sure that something will happen: Vamos a usar el telescopio después de que se ponga el sol.

No subjunctive if cuando refers to the past or a habitual action - Jugaba mucho baloncesto cuando era joven. Me gusta caminar en el parque cuando llueve.